Energy Cost Savings Calculator (Tank vs Tankless)

ANA›Life Services Authority›National Calculator Authority›Energy Cost Savings Calculator (Tank vs Tankless)

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Energy Cost Savings Calculator (Tank vs Tankless)

Compare the annual energy costs of a traditional tank water heater versus a tankless (on-demand) water heater and estimate your potential savings.

Daily Hot Water Usage (gallons)

Incoming Cold Water Temperature (°F)

Desired Hot Water Temperature (°F)

Fuel Type

Natural Gas Electric

Natural Gas Rate ($/therm)

Electricity Rate ($/kWh)

Tank Water Heater Energy Factor (UEF)

Tankless Water Heater Energy Factor (UEF)

Analysis Period (years)

Calculate Savings

function eneToggleFuel() { var fuel = document.getElementById('ene_fuel_type').value; document.getElementById('ene_gas_rate_row').style.display = (fuel === 'gas') ? '' : 'none'; document.getElementById('ene_elec_rate_row').style.display = (fuel === 'electric') ? '' : 'none'; }

function eneCalc() { var resultDiv = document.getElementById('ene_result'); resultDiv.style.display = 'none';

// --- Read inputs --- var dailyGal = parseFloat(document.getElementById('ene_daily_gallons').value); var inletTemp = parseFloat(document.getElementById('ene_inlet_temp').value); var outletTemp = parseFloat(document.getElementById('ene_outlet_temp').value); var fuel = document.getElementById('ene_fuel_type').value; var gasRate = parseFloat(document.getElementById('ene_gas_rate').value); var elecRate = parseFloat(document.getElementById('ene_elec_rate').value); var tankUEF = parseFloat(document.getElementById('ene_tank_ef').value); var tanklessUEF= parseFloat(document.getElementById('ene_tankless_ef').value); var years = parseFloat(document.getElementById('ene_years').value);

// --- Validation --- var errors = []; if (isNaN(dailyGal) || dailyGal 0 gallons."); if (isNaN(inletTemp) || inletTemp 80) errors.push("Inlet temperature must be between 32°F and 80°F."); if (isNaN(outletTemp) || outletTemp 160) errors.push("Outlet temperature must be between 90°F and 160°F."); if (outletTemp 0."); if (fuel === 'electric' && (isNaN(elecRate) || elecRate 0."); if (isNaN(tankUEF) || tankUEF 0.95) errors.push("Tank UEF must be between 0.50 and 0.95."); if (isNaN(tanklessUEF) || tanklessUEF 0.99) errors.push("Tankless UEF must be between 0.80 and 0.99."); if (tanklessUEF 30) errors.push("Analysis period must be between 1 and 30 years.");

if (errors.length > 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; resultDiv.style.display = 'block'; return; }

// --- Core Formula --- // Step 1: Daily energy needed to heat water (BTU/day) // Q_daily = gallons/day × 8.3454 lb/gal × 1 BTU/(lb·°F) × ΔT var deltaT = outletTemp - inletTemp; // °F var lbPerGal = 8.3454; // lb/gal var btuPerDay = dailyGal * lbPerGal * deltaT; // BTU/day (useful heat) var btuPerYear= btuPerDay * 365; // BTU/year

var tankAnnualCost, tanklessAnnualCost;

if (fuel === 'gas') { // 1 therm = 100,000 BTU // Annual fuel consumed = useful BTU / UEF / 100,000 therms var tankTherms = btuPerYear / tankUEF / 100000; var tanklessTherms = btuPerYear / tanklessUEF / 100000; tankAnnualCost = tankTherms * gasRate; tanklessAnnualCost = tanklessTherms * gasRate; } else { // 1 kWh = 3412.14 BTU // Annual electricity consumed = useful BTU / UEF / 3412.14 kWh var tankKwh = btuPerYear / tankUEF / 3412.14; var tanklessKwh = btuPerYear / tanklessUEF / 3412.14; tankAnnualCost = tankKwh * elecRate; tanklessAnnualCost = tanklessKwh * elecRate; }

var annualSavings = tankAnnualCost - tanklessAnnualCost; var totalSavings = annualSavings * years; var savingsPct = (annualSavings / tankAnnualCost) * 100;

// Monthly figures var tankMonthly = tankAnnualCost / 12; var tanklessMonthly = tanklessAnnualCost / 12; var monthSavings = annualSavings / 12;

function fmt(n, d) { d = (d === undefined) ? 2 : d; return n.toFixed(d); } function fmtMoney(n) { return '$' + fmt(n); }

var fuelLabel = (fuel === 'gas') ? 'Natural Gas' : 'Electric'; var savingsClass = annualSavings >= 0 ? 'calc-positive' : 'calc-negative';

resultDiv.innerHTML = '### Results ' + '' + 'MetricTank HeaterTankless Heater' + '' + 'Fuel Type' + fuelLabel + '' + 'UEF (Energy Factor)' + fmt(tankUEF,2) + '' + fmt(tanklessUEF,2) + '' + 'Useful Heat Needed (BTU/yr)' + Math.round(btuPerYear).toLocaleString() + '' + 'Monthly Energy Cost' + fmtMoney(tankMonthly) + '' + fmtMoney(tanklessMonthly) + '' + 'Annual Energy Cost' + fmtMoney(tankAnnualCost) + '' + fmtMoney(tanklessAnnualCost) + '' + '' + '' + '' + 'Monthly Savings: ' + fmtMoney(monthSavings) + '

' + 'Annual Savings: ' + fmtMoney(annualSavings) + ' (' + fmt(savingsPct,1) + '% reduction)

' + 'Total Savings over ' + years + ' year(s): ' + fmtMoney(totalSavings) + '

' + '' + (annualSavings

#### Formula

Step 1 — Daily useful heat required (BTU/day):

Q = Gallons/day × 8.3454 lb/gal × ΔT (°F)

where ΔT = Outlet Temp − Inlet Temp

Step 2 — Annual fuel consumption:

Gas: Therms/yr = (Q × 365) ÷ UEF ÷ 100,000

Electric: kWh/yr = (Q × 365) ÷ UEF ÷ 3,412.14

Step 3 — Annual energy cost:

Gas: Cost = Therms/yr × $/therm

Electric: Cost = kWh/yr × $/kWh

Step 4 — Annual savings:

Savings = Tank Cost − Tankless Cost

Total Savings = Annual Savings × Years

#### Assumptions & References

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References

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